Contact lens material

ABSTRACT

Cross-linked copolymers which are obtained by polymerising a neutral diluent monomer or monomers, a monomer or monomers bearing a center of permanent positive charge, and a bifunctional and/or trifunctional cross-linking agent, are suitable for use in contact lenses. Process for producing them by copolymerisation, contact lens material comprising them, contact lenses made of them and processes for producing contact lenses from them.

[0001] The present invention relates to copolymers, in particular suitable for use in contact lenses.

[0002] The use of synthetic hydrogels for contact lenses was first demonstrated by Wichtecte and Lim in the 1960's. Early hydrogels employed 2-hydroxyethyl methacrylate (HEMA) as principal monomer, together with some of the homologous esters of the glycol monomethacrylate series such as diethylene glycol monomethacrylate and tetraethylene glycol monomethacrylate. It was later found that slightly crosslinked copolymers of the higher glycol monomethyacrylates and 2-hydroxyethyl methacrylate yielded transparent hydrogels that swelled in water to a higher hydration than the hydrogels of 2-hydroxyethyl methacrylate.

[0003] The water content of hydroxyalkyl methacrylate based gels can be further increased by the addition of vinyl lactams, methacrylic acids, acrylic acids, acrylamides and methacrylamides. Although the required degree of gel hydration can be achieved by the addition of anionic monomers, it is well known that these gels display high levels of protein deposition on and occasionally within the gel matrix.

[0004] It has now surprisingly been found that effective contact lens materials which have both good transparency and a high degree of water swellability are provided by copolymers which have a permanent positive charge built into them. Such polymers are formed by polymerising and crosslinking a neutral diluent monomer, for example HEMA, with a co-monomer bearing a centre of permanent positive charge. These formulations have been found to have a high level of protein resistance to tear component deposition and a reduction in lens water loss.

[0005] Accordingly, the present invention provides a crosslinked copolymer which is obtainable by polymerising a neutral diluent monomer or monomers, a monomer or monomers bearing a centre of permanent positive charge, and a bifunctional or trifunctional crosslinking agent.

[0006] The crosslinked copolymers of the present invention therefore comprise residues of a diluent monomer or monomers, a monomer or monomers bearing a centre of permanent positive charge, and a bifunctional or trifunctional crosslinking agent.

[0007] The copolymers of the invention may be xerogels which do not contain any water. Alternatively, they may be in the form of hydrogels which do contain water.

[0008] The invention also provides a process for producing such a crosslinked copolymer, a contact lens material comprising such a copolymer, a contact lens made from such a copolymer, and use of such a copolymer or contact lens material in the production of a contact lens.

[0009] Diluent Comonomer

[0010] The diluent monomer can act as a solvent for the comonomers during copolymerisation to produce the copolymer if no additional solvent is present. Where the diluent monomer and monomer bearing the centre of permanent positive charges are immiscible a solvent can be used to aid mixing.

[0011] Particular examples of diluent comonomers include alkyl (alk)acrylate preferably containing 1 to 12, more preferably 1 to 4, carbon atoms in the alkyl group of the ester moiety, such as a methyl (alk)acrylate and butyl (alk)acrylate; a dialkylamino alkyl (alk)acrylate, preferably containing 1 to 4 carbon atoms in each alkyl moiety of the amine and 1 to 4 carbon atoms in the alkylene chain, e.g. 2(dimethylamino)ethyl (alk)acrylate; an alkyl (alk)acrylamide preferably containing 1 to 4 carbon atoms in the alkyl group of the amide moiety; a hydroxyalkyl (alk)acrylate preferably containing from 1 to 4 carbon atoms in the hydroxy moiety, e.g. a 2-hydroxyethyl (alk)acrylate; or a vinyl monomer such as an N-vinyl lactam, preferably containing from 5 to 7 atoms in the lactam ring for instance vinyl pyrrolidone; styrene or a styrene derivative which for example is substituted on the phenyl ring by one or more alkyl groups containing from 1 to 4 carbon atoms, and/or by one or more halogen, such as fluorine atoms.

[0012] It is to be understood that throughout the specification (alk)acrylate, (alk)acrylic and (alk)acrylamide mean acrylate or alkacrylate, acrylic or alkacrylic and acrylamide or alkacrylamide respectively. Preferably alkacrylate, alkacrylic and alkacrylamide groups contain from 1 to 4 carbon atoms in the alkyl group thereof and are most preferably methacrylate, methacrylic or methacrylamide groups. Similarly (meth)acrylate, (meth)acrylic and (meth)acrylamide shall be understood to mean acrylate or methacrylate, acrylic or methacrylic and acrylamide or methacrylamide respectively.

[0013] Preferably the diluent monomer is selected from vinylpyrrolidone, 2-hydroxyethyl methacrylate, methyl methacrylate and mixtures thereof, most preferably 2-hydroxyethyl methacrylate, methyl methacrylate and mixtures thereof. In one embodiment diluent monomers are vinylpyrrolidone, 2-hydroxyethyl methacrylate and mixtures thereof.

[0014] Comonomers Bearing A Centre of Permanent Positive Charge

[0015] The comonomer bearing the centre of permanent positive charge can either be cationic or zwitterionic. In the latter case the monomer includes within its structure not only a centre of permanent positive charge but also a centre of negative charge. Typically the centre of permanent positive charge in both cationic and zwitterionic comonomers is provided by a quaternary nitrogen atom.

[0016] Preferred comonomers which bear a centre of positive charge are of general formula (I)

Y—B—X  (I)

[0017] wherein B is a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene chain or if X contains a carbon-carbon chain between B and the centre of permanent positive charge or if Y contains a terminal carbon atom, a valence bond,

[0018] X is a group bearing a centre of permanent positive charge and

[0019] Y is an ethylenically unsaturated polymerisable group selected from

[0020] wherein:

[0021] R is hydrogen or a C₁-C₄ alkyl group;

[0022] A is —O— or —NR¹— where R¹ is hydrogen or a C₁-C₄ alkyl group or R¹ is —B—X where B and X are as defined above.

[0023] K is a group —(CH₂)_(p)OC(O)—, —(CH₂)_(p)C(O)O—, —(CH₂)_(p)OC(O)O—, —(CH₂)_(p)NR²—, —(CH₂)_(p)NR²C(O)—, —(CH₂)_(p)C(O)NR²—, (CH₂)_(p)NR²C(O)O—, —(CH₂)_(p)OC(O)NR²—, —(CH₂)_(p)NR²C(O)NR²— (in which the groups R² are the same or different), —(CH₂)_(p)O—, —(CH₂)_(p)SO₃—, or, optionally in a combination with B, a valence bond, and p is from 1 to 12 and R² is hydrogen or a C₁-C₄ alkyl group.

[0024] The proviso on whether B may be a valence bond ensures that the centre of permanent positive charge in X is not directly bonded to a heteroatom, such as an oxygen or nitrogen atom in Y.

[0025] Preferred monomers which bear a centre of positive charge are those of general formula (II) or (II).

[0026] where R. A, B and X are as defined with reference to formula (I).

[0027] Preferably R is hydrogen, methyl, or ethyl, more preferably methyl, so that the monomer of formula (II) is an acrylic acid, methacrylic acid or ethacrylic acid derivative.

[0028] In the compounds of formula (III) K may be a valence bond and B a group, K may be a group and B a valence bond, both K and B may be groups or K and B may together be a valence bond. Preferably B is a group where X is a valence bond. Where K is a group then preferably p is from 1 to 6, more preferably 1, 2 or 3 and most preferably p is 1. When K is a group —(CH₂)_(p)NR²—, —(CH₂)_(p)NR²C(O)—, —(CH₂)_(p)C(O)NR²—, —(CH₂)_(p)NR²C(O)O—, —(CH₂)_(p)OCNR²— or —(CH₂)_(p)NR²C(O)NR²— then R² is preferably hydrogen, methyl or ethyl, more preferably hydrogen.

[0029] Preferably B is:

[0030] an alkylene group of formula —(CR³ ₂)_(a)—, wherein the groups —(CR³ ₂)— are the same or different, and in each group —(CR³ ₂)— the groups R³ are the same or different and each group R³ is hydrogen or C₁₋₄ alkyl, preferably hydrogen, and a is from 1 to 12, preferably 1 to 6;

[0031] an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably —CH₂O(CH₂)₄—;

[0032] an oligo-oxaalkylene group of formula —[(CR⁴ ₂)_(b)O]_(c)(CR⁴ ₂)_(b)— where the groups —(CR⁴ ₂)— are the same or different and in each group —(CR⁴ ₂)— the groups R⁴ are the same or different and each group R⁴ is hydrogen or C₁₋₄ alkyl, preferably hydrogen, and b is 2 or 3 and c is from 2 to 11, preferably 2 to 5;

[0033] or a valence bond but only if X contains a carbon-carbon chain between B and the centre of positive charge, or if Y contains a terminal carbon atom.

[0034] Preferred groups B include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 12 carbon atoms.

[0035] Preferred groups X are the groups of formula (IVA), (IVB), (IVC), (IVD), (IVE) and (IVF) as defined below, of which the groups of formula (IVC) are particularly preferred.

[0036] The groups of formula (IVA) are:

[0037] where the groups R⁵ are the same or different and each is hydrogen or C₁₋₄ alkyl and Z^(⊖) is a counterion.

[0038] Preferably the groups R⁵ are all the same. It is also preferable that at least one of the groups R⁵ is methyl, and more preferable that all the groups R⁵ are methyl.

[0039] The counterion Z^(⊖) present in the compounds of formula (II) or (III) containing a group of formula (IVA) is such that the compounds are neutral salts. The counterion may be exchanged with ions in physiological fluids and thus the specific nature of the counterion is not critical in the present invention. However, physiologically acceptable counterions are preferred. Suitable physiologically acceptable counterions include halide anions, such as chloride, bromide or fluoride ions, other inorganic anions such as sulphate, phosphate and phosphite and organic anions such as aliphatic mono-, di- or tri-carboxylate anions containing from 2 to 25 carbon atoms and optionally bearing one or more hydroxyl groups e.g. acetate, citrate and lactate.

[0040] When X is a group of formula (IVA), preferably B is a group of formula —(CR³ ₂)— or —(CR³ ₂)₂—, eg. —(CH₂)— or —(CH₂CH₂)—.

[0041] The groups of formula (IVB) are:

[0042] where the groups R⁶ are the same or different and each is hydrogen or C₁₋₄ alkyl and d is from 2 to 4.

[0043] Preferably the groups R⁶ are the same. It is also preferable that at least one of the groups R⁶ is methyl, and more preferable that the groups R⁶ are both methyl.

[0044] Preferably d is 2 or 3, more preferably 3.

[0045] When X is a group of formula (IVB) preferably B is a group of formula —(CR³ ₂)— or —(CR³ ₂)₂—, eg. —(CH₂)— or —(CH₂CH₂)—.

[0046] The groups of formula (IVC) are:

[0047] where the groups R⁷ are the same or different and each is hydrogen or C₁₋₄ alkyl, and e is from 1 to 4.

[0048] Preferably the groups R⁷ are the same. It is also preferable that at least one of the groups R⁷ is methyl, and more preferable that the groups R⁷ are all methyl.

[0049] Preferably e is 2 or 3, more preferably 2.

[0050] When X is a group of formula (IVC) preferably B is a group of formula —(CR³ ₂)— or —(CR³ ₂)₂—, eg. —(CH₂)— or —(CH₂CH₂)—.

[0051] The groups of formula (IVD) are:

[0052] wherein the groups R⁸ are the same or different and each is hydrogen or C₁₋₄ alkyl, B¹ is a valence bond or straight or branched alkylene, oxaalkylene or oligo-oxaalkalkylene group, f is from 1 to 4 and if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom, and otherwise Z is 1.

[0053] Preferably the groups R⁸ are the same. It is also preferable that at least one of the groups R⁸ is methyl, and more preferable that the groups R⁸ are all methyl. Preferably f is 1 or 2, more preferably 2.

[0054] Preferably B¹ is:

[0055] a valence bond;

[0056] an alkylene group of formula —(CR^(3a) ₂)_(aa)—, wherein the groups —(CR^(3a) ₂)— are the same or different, and in each group —(CR^(3a) ₂)— the groups R^(3a) are the same or different and each group R^(3a) is hydrogen or C₁₋₄ alkyl, preferably hydrogen, and aa is from 1 to 24, preferably 6 to 18;

[0057] an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably —CH₂O(CH₂)₄—; or

[0058] an oligo-oxaalkylene group of formula —[(CR^(4a) ₂)_(ba)O]_(ca)— where the groups —(CR^(4a) ₂)— are the same or different and in each group —(CR^(4a) ₂)— the groups R^(4a) are the same or different and each group R^(4a) is hydrogen or C₁₋₄ alkyl, preferably hydrogen, and ba is 2 or 3 and ca is from 1 to 12, preferably 1 to 6.

[0059] Preferred groups B¹ include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 24 carbon atoms.

[0060] In one embodiment B and B¹ are the same.

[0061] The groups of formula (IVE) are:

[0062] wherein the groups R⁹ are the same or different and each is hydrogen or C₁-C₄ alkyl, B² is a valence bond or a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene group, g is from 1 to 4 and if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is 1.

[0063] Preferably the groups R⁹ are the same. It is also preferable that at least one of the groups R⁸ is methyl, and more preferable that the groups R⁸ are all methyl. Preferably g is 1 or 2, more preferably 2.

[0064] Preferably B² is:

[0065] a valence bond;

[0066] an alkylene group of formula (CR^(3b) ₂)_(ab)—, wherein the groups —(CR^(3b) ₂)— are the same or different, and in each group —(CR^(3b) ₂)— the groups R^(3b) are the same of different and each group R^(3b) is hydrogen or C₁₋₄ alkyl, preferably hydrogen, and ab is from 1 to 24, preferably 6 to 18;

[0067] an oxaalkylene group such as alkoxyalkyl having 1 to 6, carbon atoms in each alkyl moiety, more preferably 'CH₂O(CH₂)₄—; or

[0068] an oligo-oxaalkylene group of formula —[(CR^(4b) ₂)_(bb)O]_(cb)— where the groups —(CR^(4b) ₂)— are the same or different and in each group —(CR⁴ _(2b))— the groups R^(4b) are the same or different and each group R^(4b) is hydrogen or C₁₋₄ alkyl, preferably hydrogen, and bb is 2 to 6 and cb is from 1 to 12, preferably 1 to 6.

[0069] Preferred groups B² include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 24 carbon atoms.

[0070] In one embodiment B and B² are the same.

[0071] The groups of formula (IVF) are:

[0072] wherein the groups R¹⁰ are the same or different and each is hydrogen or C₁₋₄ alkyl, B³ is a valence bond or a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene group, h is from 1 to 4 if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is 1.

[0073] Preferably the groups R¹⁰ are the same. It is also preferable that at least one of the groups R¹⁰ is methyl, and more preferable that the groups R¹⁰ are all methyl.

[0074] Preferably h is 1 or 2, more preferably 2.

[0075] Preferably B³ is:

[0076] a valence bond;

[0077] an alkylene group of formula —(CR^(3c) ₂)_(ac)—, wherein the groups —(CR^(3c) ₂)— are the same or different, and in each group —(CR^(3c) ₂)— the groups R^(3c) are the same or different and each group R^(3c) is hydrogen or C₁₋₄ alkyl, preferably hydrogen, and ac is from 1 to 24, preferably 6 to 18;

[0078] an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, more preferably —CH₂O(CH₂)₄—; or

[0079] an oligo-oxaalkylene group of formula —[(CR^(4c) ₂)_(bc)O]_(cc)— where the groups —(CR^(4c) ₂)— are the same or different and in each group —(CR^(4c) ₂)— the groups R^(4c) are the same or different and each group R^(4c) is hydrogen or C₁₋₄ alkyl, preferably hydrogen, and ba is 2 to 6 and cc is from 1 to 12, preferably 1 to 6.

[0080] Preferred groups B³ include a valence bond and alkylene, oxaalkylene and oligo-oxaalkylene groups of up to 24 carbon atoms.

[0081] In one embodiment B and B³ are the same.

[0082] According to one particular embodiment, the monomer bearing a centre of permanent positive charge is a monomer of formula (V)

[0083] wherein BB is a straight or branched C₁-C₆ alkylene chain optionally interrupted by one or more oxygen atoms;

[0084] nn is from 1 to 12

[0085] R¹¹ is H or a C₁-C₄ alkyl group; and

[0086] YY is a group which includes a centre of positive charge. More preferably,

[0087] YY is a group selected from:

[0088]  the group BB in (VID) and (VIE) being a linear or branched alkylene chain as defined above and nn being as defined above.

[0089] Preferably BB is a group selected from —CH₂—, —C(R¹²)₂—, in which R¹² is C₁₋₄ alkyl, and —CH₂—CH₂—O—.

[0090] Preferably in compounds of formula (V), R¹¹ is hydrogen or methyl.

[0091] When X is a group as defined under (VID) or (VIE), the group (BB)_(nn) is preferably chosen to avoid steric hindrance in the vicinity of the adjacent —OC(O)— group, the reactivity of which could be adversely affected by such steric hindrance.

[0092] Preferred examples of co-monomers of formula (I) are:

[0093] Particular examples of preferred comonomers bearing a centre of permanent positive charge are 2(methacryloyloxy)ethyl-2′(trimethylammonium)ethyl phosphate inner salt [Compound C above] and 1[4(4′-vinylbenzyloxy)butane]-2″(trimethylammonium)ethyl phosphate inner salt [a compound of formula (III)].

[0094] Comonomers bearing a centre of permanent positive charge, such as those of formulae (II) and (III), and comonomers of formula (V) may be prepared by conventional techniques using known reactions, for example using a suitable substituted alkyl (alk)acrylate, glycerophosphoryl choline or suitable substituted styrene as starting material.

[0095] Examples of suitable substituted alkyl (alk)acrylates include dimethylaminoethyl(meth)acrylate and 2-hydroxyethyl(meth)acrylate.

[0096] Comonomers of formula (II) or (III) containing a group of formula (IVA), (IVB) or (IVC) and comonomers of formula (V) including a group of formula (VIA), (VIB), and (VIC) may be prepared as described in Reference Examples 1 to 4 or by analogous known methods.

[0097] Comonomers of formula (II) or (III) containing a group of formula (IVD) and comonomer of formula (V) including a group of formula (VID) may be prepared by selective acylation of glycerophosphorylcholine or analogues thereof at the primary hydroxyl group with an activated acid derivative such as an acid anhydride O[C(O)B¹CH₃]₂ or an acid halide CH₃B¹COHal where B¹ is as defined above and Hal is halogen, followed by acylation of the secondary hydroxyl group with an appropriate acylating agent, for example methacryloyl chloride. Purification, for example by column chromatography on a suitable support, may be performed after each acylation or after the second acylation only. Suitable activated acid derivatives include acid anhydrides, acid halides, reactive esters and imidazolides. The acylations may be performed in a suitable anhydrous, aprotic solvent, for example N,N-dimethylformamide, optionally in the presence of a suitable non-nucleophilic base, for example triethylamine.

[0098] Alternatively, the primary alcohol group in glycerophosphoryl choline or an analogue thereof may be blocked by reaction with a suitable protecting group reagent, for example t-butyldimethylsilyl chloride, under standard conditions and the secondary hydroxy group then treated with an acylating agent such as methacryloyl chloride. The t-butyldimethylsilyl protecting group may be removed by treatment with a dilute organic or mineral acid, for example p-toluene sulphonic acid, hydrochloric acid or with tetra-butylammonium fluoride. The deblocked primary hydroxyl group may then be treated with an activated acid derivative such as an acid anhydride O[C(O)B¹CH₃]₂ or acid halide CH₃B¹COHal where B¹ is as defined above, and Hal is halogen.

[0099] Analogues of glycerophosphorylcholine may be prepared by reaction of phosphorus oxychloride with a bromoalcohol in an inert aprotic solvent, such as dichloromethane, to give a bromoalkylphosphorodichloridate. The dichloro derivative thus produced may then be treated with 2,2-dimethyl 1,3-dioxolane-4-methanol in the presence of a base, for example triethylamine, followed by acid hydrolysis to give a bromoalkylphosphorogylcerol derivative. This may then be treated with an amine NR⁸ ₃, where R⁸ is as defined above, for example trimethylamine, to generate the glycerophosphorylcholine analogue. This preparation is depicted in the following scheme.

[0100] where R⁸ and f are as defined in relation to groups of formula (IVD).

[0101] Comonomers of formula (II) or (III) containing a group of formula (IVE) and comomers of formula (V) containing a group of formula (VIE) may be prepared by the selective acylation of glycerophosphorylcholine or an analogue thereof at the primary hydroxyl group with for example, methacryloyl chloride followed by reaction at the secondary hydroxyl group using an activated acid derivative, such as an acid halide O[C(O)B²CH₃]₂ or an acid halide CH₃B²COHal, where B² is as defined above and Hal is halogen. The intermediates and final products may be purified, as necessary using column chromatography. Optionally, protecting group strategy, similar to that outlined above in relation to production of comonomers containing a group of formula (IVD), may be employed.

[0102] Comonomers of formula (II) or (III) containing a group of formula (IVF) may be prepared in an analogous manner to comonomers containing groups of formula (IVD) or (IVE).

[0103] Crosslinking Comonomers

[0104] The copolymers of the invention also comprise residues of difunctional and/or trifunctional comonomers. Such comonomers are capable of crosslinking the polymer during polymerisation. Conventional crosslinking agents may be used.

[0105] Examples of suitable crosslinking comonomers include alkane diol or triol di- or tri(alk)acrylates, eg (meth)acrylates, preferably containing 1 to 8 carbon atoms in the diol or triol residue; alkylene di- or tri-(alk)acrylamides, e.g. (meth)acrylamides, preferably containing 1 to 6 carbon atoms in the alkylene group and and di- or tri-vinyl compounds such as di- or tri-vinyl benzene compounds. Particular examples of crosslinking agents include ethyleneglycoldimethacrylate, tetraethyleneglycol dimethacrylate, trimethylolpropanetrimethacrylate and N,N-methylenebisacrylamide.

[0106] Optionally the comonomer mixture used for polymerising the copolymer further comprises a gel swelling monomer such as an N-vinyl lactam, methacrylic acid or acrylic acid and where appropriate a bulking or solvating agent such as a solvent, for example, an alcohol or water.

[0107] Polymers of the invention may be prepared by copolymerising monomers bearing a centre of permanent positive change, diluent monomers and crosslinking monomers usually by bulk polymerisation in an appropriate mould. Additionally a solvent or solvent mixture may be included to provide a suitable reaction medium for immiscible comonomers. Suitable solvents include water, halogenated organic solvents and non-halogenated organic solvents. Initiators and/or reagents to modify the bulk morphology of the final polymer may also be included. Any conventional technique may be used for the polymerisation, typically thermal polymerisation or ultraviolet polymerisation.

[0108] The invention therefore further provides a method of preparing a crosslinked polymer which comprises copolymerising a monomer composition, such as a monomer solution, comprising a diluent monomer or monomers, a comonomer or comonomers including within its structure a centre of permanent positive charge, and a monomoner or monomers which will crosslink the resultant polymer. Optionally, the monomer composition further comprises a solvent or solvent mixture and a polymerisation initiator or initiators.

[0109] The monomer composition which is subjected to polymerisation typically comprises at least 30%, preferably at least 60%, and up to 99.79% by weight of diluent monomer. It typically comprises at least 0.2% and up to 50% monomer or monomers which contain a centre of permanent positive charge and from 0.01% to 20% by weight of crosslinking monomer. Optionally up to 10% by weight of gel swelling monomer is included.

[0110] In one embodiment the monomer composition which is subjected to polymerisation typically comprises at least 70%, preferably at least 80% by weight of the diluent monomer. It further comprises at least 0.2% and up to 20% of monomer or monomers which bear a centre of permanent positive charge and, optionally, up to 10% by weight of gel-swelling monomer or monomers.

[0111] The monomer composition may comprise conventional further polymer ingredients such as cross-linking agents and polymerisation initiators. These further ingredients are in one embodiment used in a total amount from 0.1 to 5%, typically from 0.2 to 3% and preferably about 0.5% by weight relative to the weight of the monomer composition prior to polymerisation.

[0112] Preferably the monomer composition comprises at least 0.01% and up to 10% of crosslinking monomer or monomers.

[0113] Examples of suitable initiators include bis(4-tertiarybutylcyclohexyl)-peroxydicarbonate, benzoylperoxide, 2,2′-azo-bis(2-methylpropionitrile) [i.e. azo-bis-isobutyro nitrile], 1-benzyl-2-hydroxy-2-dimethylethane-1-one and benzoin methylether. An initiator is generally used in a total amount from 0.1% to 5, typically from 0.2% to 3% and preferably about 0.5% by weight relative to the weight of the total monomer composition prior to polymerisation

[0114] Additionally the monomer composition may have added to it a solvent or solvent mixture. Examples of suitable solvents are ethanol, methanol and water. When present, solvent suitably comprises from 0.1 to 50 weight % of the total reaction mixture, preferably from 5 to 40 weight %.

[0115] The polymer is prepared by dissolving the monomer or monomers bearing the centre of positive charge in the diluent monomer or monomers or diluent monomer/solvent mixture together with the crosslinking monomer or monomers and if present the polymerisation initiator or initiators. The solution thus formed is then purged with nitrogen, to remove any oxygen which may be present before the polymerisation process is begun. Polymerisation is carried out in a sheet-forming mould, a contact lens precursor button (thick round disc) mould, a contact lens mould or to provide a cylindrical polymer rod. For example, when carried out in a sheet-forming mould the monomer solution may be injected between two spaced plates and then polymerised in situ to generate a polymer sheet.

[0116] Generally the copolymers of the invention will be produced by copolymerisation in the absence water. This produces a xerogel material which can be moulded into contact lenses directly or moulded to give contact lens buttons which can be lathe cut using methods known in the art to produce contact lenses. The xerogel material may be washed in water or in aqueous buffer to remove any excess monomer and initiator. The xerogel material can be subsequently hydrated to produce hydrogel with an equilibrium water content of up to 90%, and preferably from 30 to 80%.

[0117] The polymers of the invention are both transparent and water swellable and therefore suitable for use as contact lens materials. In particular, the polymer may be suitable for use in contact lenses which are for example soft or gas permeable contact lenses.

[0118] The invention further provides contact lenses made from polymers of the invention as hereinbefore defined.

[0119] The invention may be further illustrated by the following examples.

EXAMPLE 1 Formation of 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt-co-2-hydroxyethylmethacrylate-co-ethyleneglycoldimethacrylate buttons

[0120] 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt (compound C) (4.86 g) was dissolved in 2-hydroxyethylmethacrylate (14.8 g), together with ethyleneglycodimethacrylate (0.25 g) and bis(4-tertiarybutylcyclohexyl)-peroxydicarbonate (0.048 g). This solution was de-gassed with nitrogen gas and then pipetted into an open stainless steel contact lens button mould. The mould was placed in an oven in a nitrogen atmosphere at 50° C. for 1¼ hours. After this time the mould was removed. The buttons were pushed out of the mould and the reaction completed by heating at 70° C. in a vacuum oven for 24 hours. The buttons were optically clear and could be machined to make contact lenses.

EXAMPLE 2 Formation of 2(methacroyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt-co-2-hydroxyethylmethacrylate-co-ethyleneglycoldimethacrylate buttons by photopolymerization

[0121] 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt (compound C) 5.00 g was dissolved in 2-hydroxyethylmethacrylate (14.2 g) together with ethyleneglycoldimethacrylate (0.2 g), 1-benzyl-2-hydroxy-2-dimethylethane-1-one (0.2 g) and bis(4-tertiarybutylcyclohexyl)-peroxydicarbonate (0.02 g). The solution was de-gassed with N₂ and then pipetted into an open stainless steel contact lens button mould. The monomer solutions were irradiated with a 100 w/inch medium pressure, mercury vapour lamp for 2 minutes. The reaction was completed thermally at 70° C. in a vacuum oven for 24 hours. The resulting buttons were machined to make contact lenses.

EXAMPLE 3 Formation of 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt-co-methylmethacrylate-co-ethyleneglycoldimethacrylate buttons

[0122] 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt (5.76 g) was dissolved in ethanol (6.5 ml) and methylmethacrylate (7.5 g). Ethyleneglycoldimethyacrylate (0.21 g), 1-benzyl-2-hydroxy-2-dimethylethane-1-one (0.2 g) and bis(4-tertiarybutylcyclohexyl)-peroxydicarbonate (0.01 g) were added to the solution and dissolved. The resulting solution was degassed with the gas and poured into an open stainless steel contact lens button mould. The solutions were then irradiated by a 100 W/inch medium pressure mercury vapour lamp for 2 minutes. The reaction was completed thermally at 70° C. in a vacuum oven for 24 hours. The ethanol was removed from these buttons by heating at 80° C. for 48 hours in a vacuum oven. The resulting buttons were machined to make contact lenses.

EXAMPLE 4 Formation of 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt-co-methylmethacrylate-co-ethyleneglycodimethylacrylate rod

[0123] A xerogel rod (1 cm diameter×10 cm) was produced as follows:

[0124] 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt (compound C) (5.77 g) was mixed with ethanol (6.5 g), methylmethacrylate (7.4 g), ethyleneglycoldimethacrylate (0.2 g) and bis(4-tertiarybutylcyclohexyl)-peroxydicarbonate (0.03 g). The mixture was added to a polypropylene tube (1 cm diameter×10 cm ) which was sealed at one end. N₂ gas was bubbled through the solution and then a cap placed over the end of the tube. The tube was then placed in an oven at 50° C. for 1.5 hours. After this time the gelled rod of polymer was removed from the tube.

[0125] The reaction was completed at 70° C. for 24 hours. After this time the rod was cut into 1 cm cylinders. These cylinders were heated in a vacuum oven at 80° C. for 48 hours in order to removed the ethanol. The resulting buttons were machined into lenses.

EXAMPLE 5 Formation of 2(methacryloyloxyethyl)-2′(trimethylammonium) ethyl phosphate inner salt-co-2-hydroxyethylmethacrylate-co-ethyleneglycoldimethylacrylate membrane

[0126] 2(methacryloxyethyl)-2(trimethylammonium)ethyl phosphate inner salt (compound C) (3.60 g) was dissolved in 2-hydroxyethylmethacrylate (6.27 g) together with ethyleneglycoldimethacrylate (0.12 g) as a crosslinking agent and azobisisobutyronitrile (0.2 g) as a polymerisation initiator. The resultant monomer solution was then deoxygenated by bubbling nitrogen through for 5 minutes.

[0127] The monomer solution thus prepared was injected into a mould formed by two glass sheets covered by spray mounted polyethyleneterephthalate sheet and separated using a polytetrafluoroethylene spacer. Polymerisation was carried out in situ by heating the mould to 80° C. for 2 hours.

[0128] The polymer sheet thus formed was removed from the mould and swollen with water or a borate buttered saline solution at pH 7.1 to form a hydrogel sheet. The starting formulation is suitable for the mould polymerisation of soft contact lenses.

EXAMPLE 6 Formation of 2(methacryloyloxyethyl)-2′(trimethyl ammonium)ethyl phosphate inner salt-co-2-hydroxy ethyl-methacrylate-co-methylmethyacrylate-co-ethylenegycoldimethacrylate membrane

[0129] The method of Example 5 was repeated just using 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt (compound C) (3.79 g), in 2-hydroxyethyl-methacrylate (8.39 g) together with methylmethacrylate (5.04 g), ethyleneglycoldimethacrylate (0.254 g) as crosslinking agent and azobisisobutyronitrile (0.15 g) as a polymerisation initiator.

[0130] The resultant hydrogel sheet is similar to that obtained in Example 5. The starting formulation is suitable for the mould polymerisation of soft contact lenses.

EXAMPLE 7 Formation of 2-(trimethylammonium)ethyl methacrylate trifluoromethane sulphate-co-2-hydroxyethylmethacrylate-co-methylene bis-acrylamide polymer sheet

[0131] 2-(trimethylammonium)ethyl methacrylate trifluoromethanesulphonate, compound A, (0.25 g) was dissolved in hydroxyethyl methacrylate (5 g) together with methylene bis-acrylamide (25 mg) as cross-linking agent and benzoyl peroxide (25 mg) as polymerisation initiator. The resulting monomer solution was then deoxygenated by bubbling nitrogen through for 5 minutes.

[0132] The monomer solution thus prepared was injected into a mould formed by two silylated glass plates separated by a teflon spacer. Polymerisation was carried out in situ by heating the mould to 70° C. and maintaining it at that temperature for 2 hours.

[0133] The polymer sheet formed was removed from the mould and swollen with water or a saline solution to form a hydrogel sheet, which is a material suitable for forming into soft contact lenses.

EXAMPLE 8

[0134] Preparation of Further Copolymers

[0135] The method of Example 6 was repeated using, respectively, each of compounds B and C and compound types D to G prepared as described in the Reference Examples in place of compound A. The hydrogel sheet formed in each case was suitable for forming into soft contact lenses.

[0136] Mechanical Testing of Copolymers

[0137] The copolymer sheets and lenses produced may be swelled in appropriate aqueous solutions and then dehydrated by heating. The water content may be determined by weight.

[0138] Tear strength measurement may be performed by Instrom analysis using appropriate ASTM procedures. Oxygen permeability may be determined with appropriate electrodes in accordance with appropriate ASTM standards. The absorption of tear proteins by the copolymers may be measured by standard spectrophotometic techniques.

EXAMPLE 9

[0139] Lathe Cutting to Produce Contact Lenses

[0140] Buttons (as prepared in Example 1) were mounted using a low melting point wax and cut with a lathe speed of 2800 rpm to produce contact lenses. Cutting times were 1-2 seconds for 0.01 mm thickness reduction from the edge to the centre. Nitrogen may be used to cool the diamond button interface. The contact lenses produced were cleaned with petroleum ether (60-80) and polished with an oil based polish (SP2).

EXAMPLE 10

[0141] Protein Adsorption and Equilibrium Water Content Study

[0142] Two hydrogel membranes of comparable water content were prepared: membrane A (comparative) comprised of methacrylic acid (16.5 mole %), 2-hydroxyethylmethacrylate, (183.3 mole %) and ethyleneglycol dimethacrylate (0.2 mole %); membrane B according to the invention comprised of 2(methacryloyloxyethyl)-2′(trimethylammonium)ethyl phosphate inner salt (40% mole), methylmethacrylate (59 mole %) and ethylene glycol dimethacrylate (1%). Both membranes were cut into 0.9 mm discs and soaked in a buffered protein solution for 24 hours at 35° C. Control lenses were soaked in buffer solution for the same length of time. The buffer solution was the same as that of the buffered protein solution except that the bovine albumin and chicken lysozyme were not added.

[0143] The composition of the buffered protein solution was as follows: Sodium Chloride 0.85% Boric Acid 0.46% Sodium Borate (10 H₂O) 0.04% Bovine Albumin 0.39% Chicken Egg Lysozyme 0.12% Water 98.4%

[0144] The conditions chosen mimic the occular environment and are equivalent to those experienced by a contact lens during 7 days wear. The equilibrium water content was measured thermogravitmetrically and the dry weights of the membranes compared after soaking in the buffer solution and the buffered protein solution.

[0145] The equilibrium water content data and changes in dry weight equivalent to the adsorption of protein from the solution are shown below: Increase in Dry Equilibrium Water Weight (g. Protein/g Membrane Content % Polymer A at 35° C. 79.4 ± 0.6 — A at 35° C. in 75.9 ± 0.9 0.13 ± 0.04 protein solution B At 35° C. 70.5 ± 0.4 — B At 35° C. in 71.1 ± 0.5 0.01 ± 0.05 protein solution

[0146] The membrane containing 2(methacryloyloxyethyl)-2′(trimethylammonium) ethyl phosphate inner salt was found to absorb significantly less protein than a membrane material of comparable water content. The equilibrium water content also remained unchanged.

REFERENCE EXAMPLE 1 Synthesis of 2(trimethylammonium)ethylmethacrylate trifluoromethanesulphonate (Compound A)

[0147] 2(Dimethylamino)ethylmethacrylate was vacuum distilled and then dissolved in 0.1M dichloromethane. Methyltrifluoromethyl sulphonate (one molar equivalent) was added slowly to the resulting solution, the temperature of the solution being maintained throughout at 40° C. or less. The product precipitated out slowly and was recovered by filtration and washed in cold dichloromethane. The synthesis is depicted in Reaction Scheme B.

REFERENCE EXAMPLE 2 Synthesis of dimethyl(2-methacryloxyethyl)-(1(2-sulphopropyl)) ammonium betaine inner salt (Compound B)

[0148] 2(Dimethylamino)ethylmethacrylate was vacuum distilled and then dissolved in 0.1M dichloromethane. To this solution was added an equimolar amount of propane sultone. The betaine slowly precipitated out of solution and was recovered by filtration and washed with cold dichloromethane. The reaction is shown in Reaction Scheme B.

REFERENCE EXAMPLE 3 Preparation of 2(methacryloyloxyethyl-2′(trimethylammonium ethyl phosphate inner salt (Compound C)

[0149] The preparation is illustrated by the reaction scheme C which follows.

[0150] a) 2-Chloro-1,3-dioxaphospholane (1)

[0151] In a flask fitted with a pressure equalising dropping funnel, reflux condenser (fitted with a CaCl₂ guard tube) and magnetic stirrer, was placed a solution of phosphorus trichloride (220 ml; 346.3 g; 2.52 mol) in dichloromethane (500 ml). Ethylene glycol (139 ml; 154.7 g, 2.49 mol) was then added dropwise via the dropping funnel at such a rate that the evolution of HCl did not become too excessive. On the addition of the ethylene glycol, the condenser was arranged for distillation, and the dichloromethane removed at atmospheric pressure. When the distillate temperature reached 60° C. the flask was arranged for vacuum distillation using a water pump, Distillation then gave 2-chloro-1,3-dioxaphospholane (158 ml; 224.5 g; 71.3%) as a colourless mobile liquid (which fumes in moist air) b.pt. 36-40° C./21 mm Hg. [cf 45.5-47° C./20 mm Hg, Lucas et al, J. Am. Chem. Soc., 72, 5491, (1950)].

[0152] IR (cm⁻¹, thin film) 2980, 2905, 1470, 1210, 1005, 930, 813, 770.

[0153] b) 2-Chloro-2-oxo-1,3,2-dioxaphospholane (2)

[0154] In a flask fitted with a magnetic stirrer, reflux condenser (fitted with a CaCl₂ guard tube) and sintered glass gas inlet tube, was placed a solution of 2-chloro-1,3-2-dioxaphospholane (100.8 g; 0.797 mol) in dry benzene (200 ml). The solution was stirred and a steady stream of oxygen was bubbled through the solution. The reaction was mildly exothermic, and temperature control was achieved by allowing the solvent to reflux. The oxygen was passed through the reaction mixture for 6 hours. The solvent was removed by rotary evaporation, and the colourless mobile residue distilled to give 2-chloro-2-oxo-1,3,2-dioxaphospholane (2) (87.41 g; 77%) as a colourless mobile liquid—b.pt 95-97° C./0.2 mbar [c.f. 102.5-105° C./1 mbar (Edmundson, Chem. Ind. (London)), 1828 (1962); 79° C./0.4 mbar (Umeda et al., Makromaol. Chem. Rapid Communo. 3, 457, (1982)].

[0155] IR(cm⁻¹, thin film) 2990, 2910, 1475, 1370, 1310, 1220, 1030, 930, 865, 830.

[0156] c) 2(2-Oxo-1,3,2-dioxaphospholan-2-yloxy)ethyl methacrylate (3)

[0157] In a flask fitted with a magnetic stirrer, low temperature thermometer, and a pressure equalising funnel fitted with a silica gel guard tube, was placed a solution of 2-hydroxyethylmethacrylate (20.00 g, 0.154 mol) and triethylamine (15.60 g; 0.154 mol ) in dry diethyl ether (300 ml). The solution was stirred and cooled to between −20° C. and −30° C. A solution of freshly distilled 2-chloro-2-oxo-1,3,2-dioxaphospholane(2) (21.9 g; 0.154 mol) in dry diethyl ether (20 ml) was then added dropwise over 30 minutes, the temperature being held at −20° C. during the addition. Stirring was continued at this temperature for a further 1 hour and then for a further hour as the reaction mixture was allowed to warm to room temperature. The precipitated triethylamine hydrochloride was removed by filtration, and was washed well with dry ether. The ether was removed from the combined filtrate and washings by rotary evaporation. The cloudy oil residue was then shaken for 5 minutes with dry diethyl ether (50 ml) to precipitate a further crop of triethylamine hydrochloride, which was again removed by filtration. Removal of the ether on the rotary evaporator gave (3) (34.18 g; 94.3%) as a colourless viscous oil.

[0158] IR (cm⁻¹ thin film) 1720, 1640, 1450, 1360, 1310, 1290, 1170, 1030, 930, 850.

[0159] NMR (CDCl₃; 60 MHz, δ ppm) 1.95 (s,3H), 4.25-4.70 (m,8H), 5.70 (m, 1H), 6.25 (m, 1H).

[0160] Rf (SiO₂, eluting with 10% MeOH:90% CH₂Cl₂ −0.9; spot visualised with molybdenum blue spray reagent (eg Sigma), and with iodine vapour).

[0161] d) 2(Methyacryloyloxyethyl)-2(trimethylammonium)ethyl phosphate inner salt (4).

[0162] The phospholane (3) (67.20 g; 0.285 mol was dissolved in 100 ml of dry acetonitrile, and placed in a heavy walled tissue culture bottle. The phospholane solution was then treated with a solution of anhydrous trimethylamine (25.74 g; 0.436 mol) in dry acetonitrile (100 ml). The vessel was then sealed, and placed in a water bath held at 50° C. for 30 hours. The vessel was opened, and the solution brought to the boil. The solution was filtered whilst hot, and then set aside for crystallisation.

[0163] The product was collected by filtration, and most of the solvent removed by suction. The wet product was then washed thoroughly with anhydrous ether, then dried in vacuo, to give (4) as a white amorphous, hygroscopic solid (51.16 g; 61%). Evaporation of the mother liquor gave a very viscous oil (20.00 g; 23%), from which further product (4) crystallised on standing at −20° C. TLC (silica gel plates, eluting with MeOH/CH₂Cl₂ (1:1 v/v)) showed one spot Rf 0.1, which was revealed with Dragendorffs reagent, Molybdenum blue spray reagent, and iodine vapour.

[0164] IR(cm⁻¹ 1720, 1640, 1320, 1300, 1230, 1170, 970, 750. NMR (D₂O; 60 MHz; δ ppm) 2.0 (s,3H), 3.27 (s,9H) 3.60-4.50 (m, 8H), 5.80, (m, 1H) and 6.25 (m, 1H). CHN Found: C, 42.98%; H, 7.88%; N, 4.42%; P, 10.51%. CHN Theory: C, 44.75%; H, 7.46%; N, 4.75%; P, 10.51%.

[0165] (d1) 2-(Methacryloyloxyethyl)-2′-trimethylammonium)ethyl phosphate inner salt [Alternative Preparation]

[0166] Into a glass pressure bottle (300 cm³), were placed 2-(2-oxo-1,3,2-dioxaphospholan-2-yloxy)ethyl methacrylate (10.0 g, 42 mmol) prepared in step (c) and dry acetonitrile (60 cm³). The pressure bottle was cooled in cold water and then trimethylamine (2.5 g, 42 mmol) was rapidly added to the cold solution. The pressure bottle was closed and then shaken in a thermostat maintained at 55° C. for 2 hours. It was then allowed to come to room temperature and to stand overnight, and was shaken again at 55° C. for 13 hours. After the reaction it was cooled down in water to 10° C. It was rapidly filtered with filter paper. The filtrate was evaporated under reduced pressure with a stream of nitrogen for 2 hours to afford the product (12.3 g, 98%) as a colourless viscous liquid which crystallised on standing in a freezer. The product could be purified by preparative liquid chromatography.

REFERENCE EXAMPLE 4 1-alkanoyl-2-methacroyl phosphatidyl choline and 1-metharyloyl-2-alkanoyl phosphatidyl choline (Compound types D and E)

[0167] Glycerophosphorylcholine (0.01 mole), obtained by base hydrolysis of natural phosphatidylcholine, may be stirred with alkynoic acid anhydride (0.01 mole) and dimethylamino pyridine (0.01 mole) in dimethylsulphoxide (150 cm³).

[0168] At the conclusion of this reaction further dimethylamino pyridine (1 mole) together with methacrylic acid anhydride (1 mole) may be added. The resulting mixture may be stirred for 24 hours. The phosphatidylcholine formed may be purified by column chromatography on silica using a gradient elution procedure with chloroform:methanol:water.

[0169] The synthesis is depicted in reaction scheme D in which R=CH₃.

REFERENCE EXAMPLE 5 Synthesis of 1-alkanoyl-2-acroyl phosphatidycholine and 1-acroyl-2-alkanoyl phosphatidylcholine (Compounds type F and G)

[0170] The procedure of Reference Example 4 may be repeated, but with acrylic acid anhydride (1 mole) being used in place of methacrylic acid anhydride. The synthesis is depicted in Reaction Scheme D in which R=H.

REFERENCE EXAMPLE 6 Preparation of 1[4(4′-vinylbenzyloxy)butane]-2″-(trimethylammonium)ethyl phospate inner salt

[0171] The synthesis is depicted in Reaction Scheme E.

[0172] 4-Hydroxy-1(4′-vinylbenzyloxy)butane (5)

[0173] 1,4-Butanediol (50.00 g) was dissolved in dry toluene (60 ml), para-choloromethylstyrene (15.62 g; 0.1 mol) was then added with stirring. A catalytic quantity of 18-crown-6 (0.3 g) was then added. The flask was stoppered, stirred at room temperature for 18 hours and for a further 4 hours at 45-60°. The resulting solution was then poured in to water (500 ml) and extracted with dichloromethane (3×75 ml). The combined extracts were dried (MgSO₄) and evaporated (20°/21 mm ) to give a yellow oil, which was distilled to give a yellow oil (14.33 g; 69.6%).b.pt. 152-157°/1 mbar. NMR (60 MHz: CDCl₃) 1.55 (m, 4H); 3.50 (m, 5H, 1H exch); 4.45, (s, 2H) 5.50 (dd, 2H), 6.75 (dd, 1H), 7.40 (m, 4H). IR (thin film), 3402, 2938, 2888, 1631, 1602, 1582, 1511, 1480, 1445, 1382, 1320, 1116, 1063, 920, 907, 827, 801, 716 and 667 cm⁻¹.

[0174] 4(2-Oxo-1,3,2-dioxaphospholane-2-yloxy)-1(4′-vinylbenyloxy)butane (6)

[0175] 4-Hydroxy-1(4′-vinylbenzyloxy)butane (5) (10.03 g; 48.69 mmol) and dried triethylamine (4.92 g, 48.69 mmol) were dissolved in dry diethyl ether (150 ml) and the resulting solution placed in a rigorously dried flask. The solution was cooled to −30° and 2-chloro-2-oxo-1,3,2-dioxaphospholane (6.94 g; 48.69 mmol) added dropwise over 30 minutes, the temperature being held at −30°. The reaction mixture was then stirred for a further 2 hours, during which time the temperature was allowed to rise to 10°. The mixture was filtered and the precipitate washed with dry ether. The filtrate was evaporated (20°/21 mm ) to give a cloudy oil. The residue was shaken with 50 ml of dry ether and refiltered. Evaporation of the filtrate gave the product as a viscous yellow oil (13.73 g; 90.4%).

[0176] TLC (eluting with 10% MeOH/90% dichloromethane) showed one major spot, which stained with acid molybdate reagent (Rf 0.61), IR (thin film) 3458, 2945, 2917, 2860, 1630, 1602, 1581, 1475, 1419, 1363, 1283, 1103, 1032, 820, 842, 807, 800, 715, 610 and 421 cm⁻¹.

[0177] 1[4(4′-Vinylbenzyloxy)butane]-2″(trimethylammonium)ethyl phosphate inner salt (7)

[0178] Trimethylamine (2.00 g, 33.9 mmol) was distilled into a reaction vessel, and frozen with liquid nitrogen. A solution of the 4(2-oxo-1,3,2-dioxaphospholane-2-yloxy)-1-(4′-vinylbenyloxy)butane (6) (10.00 g, 32.1 mmol) in anhydrous acetonitrile (40 ml) was then added to the reaction vessel, which was then sealed and placed in a thermostatted water bath (50° for 50 hours). The reaction vessel was then cooled to room temperature, opened, and the reaction mixture evaporated to about half its original volume (21 mm pressure). The concentrated solution was then stirred at room temperature, whilst anhydrous ether (200 ml) was added dropwise to precipitate the product as a viscous oil. The mixture was then left for several hours at −10°. The product was collected by decanting off the supernatent solid. TLC (eluting with methanol/dichloromethane 1:1) showed one major spot at Rf 0.0-0.1 which stained with both Dragendorffs reagent and acid molybdate.

[0179] Steps (a) to (d) correspond with the steps in Reference Example 4 

1. A crosslinked copolymer which is obtainable by polymerising a neutral diluent monomer or monomers, a zwitterionic monomer or monomers and a bifunctional or trifunctional crosslinking agent.
 2. A copolymer according to claim 1 in which the diluent monomer is selected from alkyl (alk)acrylates, dialkylamino alkyl (alk)acrylates, alkyl (alk)acrylamides hydroxyalkyl (alk)acrylates, N-vinyl lactams, styrene, substituted styrene derivatives; and mixtures thereof.
 3. A copolymer according to claim 2 in which the diluent monomer is selected from vinylpyrrolidone, 2-hydroxyethylmethacrylate, methylmethacrylate and mixtures thereof.
 4. A copolymer according to any one of the preceding claims in which the zwitterionic comonomer or comonomers bears a centre of positive charge provided by a quaternary nitrogen atom.
 5. A copolymer according to any one of the preceding claims which is obtainable by copolymerising a zwitterionic monomer of formula (I) Y—B—X  (I) wherein B is a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene chain or if X contains a carbon-carbon chain between B and the zwitterionic group or if Y contains a terminal carbon atom, a valence bond, X is a zwitterionic group and Y is an ethylenically unsaturated polymerisable group selected from

wherein: R is hydrogen or a C₁-C₄ alkyl group; A is —O— or —NR¹— where R¹ is hydrogen or a C₁-C₄ alkyl group or R¹ is —B—X where B and X are as defined above; and K is a group —(CH₂)_(p)OC(O)—, —(CH₂)_(p)C (O)O—, —(CH₂)_(p)OC(O)O—, —(CH₂)_(p)NR²—, —(CH₂)_(p)NR²C(O)—, —(CH₂)_(p)C(O)NR²—, (CH₂)_(p)NR²C(O)O—, —(CH₂)_(p)OC(O)NR²—, —(CH₂)_(p)NR²C(O)NR²— (in which the groups R² are the same or different), —(CH₂)_(p)O—, —(CH₂)_(p)SO₃—, or, optionally in a combination with B, a valence bond, and p is from 1 to 12 and R² is hydrogen or a C₁-C₄ alkyl group.
 6. A copolymer according to claim 5 in which B is an alkylene group of formula —(CR³ ₂)_(a)—, wherein the groups —(CR³ ₂)— are the same or different, and in each group —(CR³ ₂)— the groups R³ are the same or different and each group R³ is hydrogen or C₁-C₄ alkyl, and a is from 1 to 12; an oxaalkylene group such as alkoxyalkyl having 1 to 6 carbon atoms in each alkyl moiety, an oligo-oxaalkylene group of formula —[(CR⁴ ₂)_(b)O]_(c)(CR⁴ ₂)_(b)— where the groups —(CR⁴ ₂)— are the same or different and in each group —(CR⁴ ₂)— the groups R⁴ are the same or different and each group R⁴ is hydrogen or C₁-C₄ alkyl, and b is 2 or 3 and c is from 2 to 11, or if X contains a carbon-carbon chain between B and the centre of positive charge, or if Y contains a terminal carbon atom, a valence bond.
 7. A copolymer according to claim 5 or 6 in which X is a group of formula (IVB):

where the groups R⁶ are the same or different and each is hydrogen or C₁₋₄ alkyl and d is from 2 to 4; a group of formula (IVC):

 where the groups R⁷ are the same or different and each is hydrogen or C₁₋₄ alkyl, and e is from 1 to 4; a group of formula (IVD):

 wherein the groups R⁸ are the same or different and each is hydrogen or C₁₋₄ alkyl, B¹ is a valence bond or straight or branched alkylene, oxaalkylene or oligo-oxalkalkylene group, f is from 1 to 4 and if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is 1; a group of formula (IVE):

 wherein the groups R⁹ are the same or different and each is hydrogen or C₁₋₄ alkyl, B² is a valence bond or a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene group, g is from 1 to 4 and if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is 1; or a group of formula (IVF):

 wherein the groups R¹⁰ are the same or different and each is hydrogen or C₁₋₄ alkyl, B³ is a valence bond or a straight or branched alkylene, oxaalkylene or oligo-oxaalkylene group, h is from 1 to 4 if B is other than a valence bond, Z is 1 and if B is a valence bond Z is 0 if X is directly bonded to an oxygen or nitrogen atom and otherwise Z is
 1. 8. A copolymer according to claim 7 in which X is a group of formula (IVD), (IVE) or (IVF) and B¹, B² or B³ respectively contains up to 24 carbon atoms.
 9. A copolymer according to claim 7 in which X is a group of formula (IVB) or (IVC).
 10. A copolymer according to claim 9, in which the group X is a group of formula (IVC).
 11. A copolymer according to claim 10 wherein the groups R⁷ are all methyl.
 12. A copolymer according to claim 11 which comprises residues of 2(methacryloyloxy)ethyl-2′-(trimethylammonium) ethyl phosphate inner salt.
 13. A contact lens material comprising a copolymer according to any one of claims 1 to
 12. 14. A contact lens comprising a copolymer according to any one of claims 1 to 12 or a contact lens material according to claim
 13. 15. A process for producing a copolymer claimed in any one of claims 1 to 12 which comprises copolymerising a monomer composition comprising a diluent monomer or monomers, a comonomer or comonomers bearing a centre of permanent positive charge, and a monomer or monomers which will crosslink the resultant polymers.
 16. Use of a copolymer according to any one of claims 1 to 12 or a contact lens material according to claim 13 in the production of a contact lens. 